This invention relates to a process for the preparation of a supramolecular polymer comprising a 4H-unit that is capable of forming at least four H-bridges in a row, preferably with another 4H-unit, leading to physical interactions between different polymer chains. The physical interactions originate from multiple hydrogen bonding interactions (supramolecular interactions) between individual 4H-units or between a 4H-unit and another moiety capable of forming hydrogen bonds thereby forming self-complementary units, preferably comprising at least four hydrogen bonds in a row. Units capable of forming at least four hydrogen bonds in a row, i.e. quadruple hydrogen bonding units, are in this patent application abbreviated as “4H-units”. Sijbesma et al. (U.S. Pat. No. 6,320,018; Science 278, 1601-1604, 1997; both incorporated by reference herein) discloses 4H-units that are based on 2-ureido-4-pyrimidones. These 2-ureido-4-pyrimidones in their turn are derived from isocytosines.
Telechelic polymers or trifunctional polymers have been modified with 4H-units (Folmer, B. J. B. et al., Adv. Mater. 12, 874-878, 2000; Hirschberg et al., Macromolecules 32, 2696-2705, 1999; Lange, R. F. M. et al, J. Polym. Sci. Part A, 37, 3657-3670, 1999; all incorporated by reference). However, these polymers are obtained by addition of solid reactants to chloroform or toluene solutions, which are both toxic organic solvents, and need prolonged reaction times of several hours in order to reach completion.
US 2004/087755, incorporated by reference, discloses polyurethane based polymers with 4H-units as end-cappers that can be used as hot melt adhesive. Example 4 in this patent discloses the preparation of supramolecular polyurethane polymers which are obtained by the bulk reaction of 6-methyl-isocytosine with 4,4′-methylene bis(phenyl isocyanate) (MDI) end-capped polyesters in the melt at 180° C., said reaction being performed in a Brabender mixer with a residence time of not more than 3 minutes. In this process it is preferred that the 6-methyl-isocytosine is added as a powder that is finely milled to a particular particle size to facilitate rapid and efficient conversion.
JP 2004250623, incorporated by reference, discloses polyester diols derived from poly(butanediol terephthalate) or polylactide that are reacted in the melt with a solid reactant comprising isocyanato functional 4H-unit, obtained by the reaction of a diisocyanate with 6-methyl-isocytosine. The reaction proceeds by kneading at 150° C. to 300° C., preferably at 160° C. to 250° C. and more preferably at 180° C. to 230° C. JP 2004250623 further discloses that it is desirable to perform the reaction above the melting point of the polymer. However, in order to control decomposition of the reactants and final products, the reaction is desirably performed at a temperature as low as possible, provided that the reactants are prevented to solidify as much as possible during the reaction. According to the examples, the reaction requires temperatures of 200° C. or higher and an excess of the isocyanato functional 4H-unit. Comparable functionalization of poly(butanediol terephthalate) and poly(butanediol isophthalate) with this isocyanato functional 4H-unit at temperatures above 180° C. are also disclosed by Yamauchi et al. (Macromolecules 37, 3519-3522, 2004; incorporated by reference). In both cases the excess of the 4H-unit in the synthesis has been removed using organic solvents (Soxhlet-extraction with methanol or precipitation from HFIP), thereby re-introducing the need of (toxic) organic solvents into the process. Moreover, the occurrence of side reactions with the isocyanate functional compound, like allophonate, biuret or isocyanurate formation, is eminent at the temperatures applied as is well known in the art (High Polymers Vol. XVI, Polyurethanes: chemistry and technology, Part 1, Ed.: J. B. Saunders and K. C. Frisch; J. Wiley & Sons, 1962; incorporated by reference).
Additionally, US 2010/0076147, incorporated by reference, discloses supramolecular polymers comprising 4H-units which are obtained by reactive extrusion in the melt at temperatures below 150° C. In order to be able to perform the melt processing, the 4H-units have been modified with e.g. C2-C20 alkyl chains on the heterocyclic ring structure in order to lower their melting point. Therefore, only specific, synthetically demanding, 4H-units can be used in this approach. Moreover, the 4H-unit is a powder at handling temperatures and needs relatively high processing temperatures (Examples 15-18 disclose reaction temperatures of 120° C. to 140° C. for the conversion into a supramolecular polymer.
Clearly, there is a need in the art for a process for the preparation of a supramolecular polymer containing a 4H-unit that does not require one or more organic solvents because of toxicological, ecological and economical reasons. Moreover, there is a need in the art for a bulk process that can be performed at temperatures below about 100° C. in which the different ingredients are formulated as liquids and can be dosed using known liquid-handling techniques in the art. There is also a need in the art for a broad range of liquid formulations comprising reactive 4H-units thereby facilitating essentially solvent-free processing of liquid reactants at moderate temperatures.